Modern ion exchange membranes are a compromise between, on the one hand, ion exchange capacity and permselectivity and, on the other, physical integrity of the membrane. The compromise is a very critical one allowing the membrane manufacturer little latitude. The "normality" or concentration of ion exchange sites in such existing membranes is from about 0.05 to 0.1N. Stated another way, the useful existing membranes contain about one covalently-bonded ionic site for about every 1200 molecular weight units. When the concentration of covalently-bonded ionic sites is increased by a very small amount to about one ion site for about 1100 molecular weight units, the membrane becomes much weaker in physical strength, it swells much too highly in water for many uses, loses some of its resistance to chemical degradation, and is otherwise unsuited to many uses under rigorous environments.
Anion exchange membranes have, in general, not been as satisfactory as cation exchange membranes. There is a particular need for anion exchange membranes of improved all around performance.
With a normality of only about 0.1, most existing membranes exhibit limited ion transfer rates, have a high electrical resistivity which may climb catastrophically in environments of high ion concentration, are easily penetrated and/or coated by nonionized metal salts, and have low permselectivities which permit cross-contamination of the two solutions separated by the membrane.
When existing cation exchange members are utilized as a divider in electrolytic generation of chlorine, the high anolyte alkali metal chloride concentrations required cause coating and/or penetration of the membrane by alkali metal chloride resulting in high cell voltage requirements.
Anion exchange membranes such as are utilized in dialysis cells have low rates of cation transfer and appear especially prone to becoming coated or fouled with deposits of metal salts, particularly salts of polyvalent metals such as may be present in the solution even in low concentrations as trace impurities. When this happens, cell operation is impeded and shutdown is required to remove the deposits from the membrane.
Water softening processes based on ion exchange resins and sea water desalinization processes are similarly low in efficiency because of low ion transfer rates and reduction in exchange efficiency due to polyvalent metal salt coatings, and penetration and plugging of the ion exchange pathways of the ion exchange material by salt-type materials.
The art of ion exchange resins is very badly in need of ion exchange materials in both granular and membrane forms having the ability to transport and/or take up the desired ion at high rates while essentially completely rejecting both nonionized salts and the unwanted coions at higher ion concentrations, i.e., cation exchange membranes which can essentially completely reject anions at higher ion concentrations yet which transport the desired cations at materially higher rates than existing membranes and anion exchange membranes which reject cations at higher ion concentrations while transporting the desired anions at materially higher rates than existing membranes.
The art of ion exchange membranes is in particular need of membranes efficiently operable under high ion concentrations near saturation and which can maintain physical integrity at high temperatures and is highly corrosive media.